Pressure-Induced Charge Disorder-Order Transition in the Cs₄O₆ Sesquioxide.

Area of Science:

• Solid-state chemistry
• Materials science
• Crystallography

Background:

• Cesium hexoxide (Cs4O6) exists in two main crystal structures: a low-temperature tetragonal phase with charge and orbital order, and a high-temperature cubic phase with charge disorder.
• These phases differ in their dioxygen anion arrangements, with the tetragonal phase containing distinct peroxide (O2^2-) and superoxide (O2^-) units, while the cubic phase has averaged dioxygen units.

Purpose of the Study:

• To investigate the pressure-induced structural transformations of Cs4O6 at both ambient and cryogenic temperatures.
• To understand the influence of pressure on the charge-disordered to charge-ordered phase transition in Cs4O6.

Main Methods:

• Synchrotron X-ray powder diffraction was employed to study the crystal structure of Cs4O6.
• Experiments were conducted at 300 K (ambient temperature) and 13.4 K (cryogenic temperature) up to pressures of approximately 12 GPa.

Main Results:

• At 300 K, increasing pressure to ~2 GPa initiated an incomplete phase transition to a denser, anisotropic tetragonal structure. A complete transformation to the charge-ordered tetragonal phase was observed above 6 GPa.
• At 13.4 K, the cubic and tetragonal phases coexisted over the entire pressure range studied (~12 GPa), with minimal conversion of the cubic to the tetragonal phase.
• Pressure was found to be an inefficient stimulus for the charge disorder-order transition at cryogenic temperatures, likely due to high activation barriers and steric hindrance.

Conclusions:

• The charge disorder-order transition in Cs4O6 is highly sensitive to temperature, with pressure being effective at ambient temperatures but not at cryogenic conditions.
• The steric hindrance for molecular oxygen unit rotation presents significant activation barriers at low temperatures, impeding pressure-induced structural and charge ordering.